They are usually rated in W and VA.

Watts is the power capacity, you just need enough for your node (probably not much, check the specs) and whatever else you plan to plug in (such as modem etc). Add everything up and ensure some % of overage.

VA is the size of the battery. The higher VA the longer it will last in case of a power outage. If you’re just looking mostly for protection against over/under current a smaller size is fine. That will protect the hardware and allow you enough time to safely shutdown. If you want it to run for a few hours to stay online in case of extended outage, consider a higher VA rating.